SUPPLEMENT 143 



Again, KNIEP (1907) has shown that when the convergent lenses of the epi- 

 dermis are transformed into dispersing lenses by smearing the epidermis over 

 with paraffin oil the leaves react heliotropically just as before, although the 

 centre of the cell in the rest position is darker than the margin. HABERLANDT 

 (1906-7) has entirely neutralized the lens action of the epidermis by covering 

 the leaf with water, and found that the heliotropic sensitivity of certain leaves 

 was then inhibited. Further research has still to show, however, whether this 

 was due to the injurious effect of the water or to the suspension of the action 

 of the lenses. 



Whatever may be the special stimulative impact that induces heliotropic 

 curvature, this stimulus must have operated for a certain time before cur- 

 vature follows as an after-effect. Here, as in geotropism, we can recognize 

 a presentation period. According to CZAPEK (1898 a) it amounts to 7 minutes 

 in the cotyledons of Avena and in Phycomyces, 10 minutes in the hypocotyls 

 of Sinapis alba and Beta vulgaris, 20 minutes in the. hypocotyl of Helianthus, 

 and 50 minutes in the epicotyl of Phaseolus. Since, however, CZAPEK'S esti- 

 mates have been shown to be too high as regards the geotropic presentation 

 period, so the same may be true here also ; nor has it been determined whether 

 and to what extent the presentation period is dependent on the intensity of 

 light. According to PFEFFER (Phys. Ill, p. 210) flashes of light of the very 

 shortest duration but of sufficient intensity lead ultimately to heliotropic 

 curvature by summation, and the pauses between individual stimuli may be 

 in this case 15-30 times as long as the stimuli. According to WIESNER (1880) 

 the sum of the individual stimuli may even be less than the presentation period 

 of the continuous stimulus that is, the intermittent stimulation may be more 

 effective than the continuous ; but this wants confirmation. 



Although, as we have already seen, geotropic differs from heliotropic per- 

 ception, we cannot say the same of the excitation which is induced after the 

 expiry of the presentation period. It is quite possible that it may be identical 

 for the two stimuli, just as the process of curvature is identical in both cases. 

 How transference of the excitation is brought about is as yet unknown. 

 All that need be said as to the path followed has been indicated above, 

 still it may be added that it is restricted to one direction, viz. towards 

 the base. 



Having now discussed heliotropic and geotropic movements separately, 

 we have still to glance at movements which result from the simultaneous or 

 rapidly consecutive stimuli of light and gravity. It must be noted more 

 especially that the plant frequently does not assume the orientation which 

 should be arrived at as a resultant of antagonistic geotropic and heliotropic 

 curvature. This fact may be illustrated by a few examples (CZAPEK, 1895). 



477, 1. 38, after falls read horizontally 



478, 11. 4-30, for It is obvious . . . presented by them, read These results show 

 that differences in the plants in the first instance make themselves apparent, 

 so that obviously in some cases geotropism, in other cases heliotropism is pre- 

 dominant. This is perfectly conceivable, for we already know that sensitivity 

 to gravity may be affected by light, and so also heliotropic sensitivity might 

 be altered by the action of gravity. Quite other factors still, however, play 

 a part in the positions of equilibrium that have been observed. Thus especially 

 there is the cessation of geoperception induced by the air of a laboratory. 

 MOLISCH'S (1905) and RICHTER'S (1906) experiments which have demonstrated 

 this effect render necessary in our opinion an entirely fresh investigation, 

 from top to bottom, of the whole problem of the simultaneous action of geo- 

 tropism and heliotropism. That CZAPEK (1898), to whom we owe the latest 



